Maleic anhydride (C2H2(CO)2O) is an important intermediate product in several large-scale industrial applications related for example to polymers and coatings. Industrial preparation of maleic anhydride is usually carried out inside tubular reactors in which the partial oxidation of n-butane with air takes place in presence of a catalyst in order to accelerate the reaction and hence to increase the reaction yield, i.e. the amount of maleic anhydride obtained. The catalyst usually comprises vanadium pentoxide and phosphorus pentoxide, which are rather expensive. Therefore, a main goal in the industrial production of maleic anhydride is to achieve a high reaction yield over a long period of time while simultaneously ensuring a long life-cycle of the catalyst.
Maleic anhydride is typically produced by means of vapor-phase partial oxidation of n-butane (C4H10) with air according to the reaction:C4H10+3.5O2→C4H2O3+4H2O  (1)
However, several parallel reactions inevitably take place, in particular the following reactions:C4H10+4.5O2→4CO+5H2OC4H10+6.5O2→4CO2+5H2OC4H10+2.5O2→2CH3COOH+H2OC4H10+2.5O2→4/3CH2CHCOOH+7/3H2O
The net production of maleic anhydride depends on the conversion rate of n-butane on the one hand, that is, the amount of n-butane oxidising, and the reaction selectivity on the other hand, that is, the relative amount of n-butane reacting according to reaction (1) leading to the production of maleic anhydride. Both the conversion rate and the reaction selectivity strongly depend on temperature and on the throughput of n-butane.
All of the above reactions are highly exothermic, although to different extents. The conversion of n-butane into maleic anhydride according to reaction (1) releases for example about 350 kcal/mole, whereas the conversion of n-butane into carbon dioxide releases about 650 kcal/mole. When the above reactions take place inside a reactor, the resulting reaction heat therefore needs to be dissipated. For this purpose, a heat transfer agent, typically a liquid coolant, is made to flow through the reactor at a temperature and flow rate appropriate for absorbing the reaction heat generated inside the reactor.
A typical tubular reactor 100 for producing maleic anhydride known in the prior art is shown in FIG. 1. The tubular reactor 100 has a cylindrical shape and a circular cross-section with a hole in the middle, such that the reactor 100 has a toroidal geometry. A plurality of tubes, typically between 5000 and 30,000 tubes are arranged between the inner wall and the outer wall of the toroid and are configured for being filled with a catalyst. In FIG. 1, only three tubes 124 of said plurality exemplary shown on illustrative purposes. The tubes 124 cross the reactor 100 in vertical direction and communicate with one or more inlets 1, through which the reacting gas mixture can enter the reactor, at one of their ends and with one or more outlets 18, through which the reacted gas mixture can exit the reactor, at the other one of their ends. A circulation pumping arrangement 22 drives a flow of a liquid coolant through the reactor 100 among the tubes 124. After circulating through the reactor 100, the liquid coolant is circulated through a heat exchanger 21 that cools down the liquid coolant before the liquid coolant re-enters the reactor to undergo a further cooling cycle.
In the tubular reactor 100 of FIG. 1, the liquid coolant generally flows in the same direction as the reacting gas mixture, which in the figure corresponds to the upward vertical direction. The flow is however additionally directed in the radial direction of the reactor alternatingly inwards and outwards by a plurality of baffles 6, 7, 8, 9, 12, 13, and 14 in order to distribute the flow of liquid coolant such that the flow of liquid coolant reaches each of the plurality of tubes 124 in which the reaction takes place.
In general, prior art reactors are designed such that most of the reaction heat is generated in a region of the reactor close to the gas inlet 1. This is so because when the reacting gas mixture is in this zone it may still be in the conditions it has been set to be, and which will usually have been chosen so as to optimize the reaction yield. However at this initial stage the reacting gas mixture has not been heated up by reaction heat yet. Therefore, as the oxidation of n-butane sets in, temperature starts increasing as a function of axial distance inside the reactor in the direction of flow of the reacting gas mixture. A typical spatial temperature profile, like the one shown in FIG. 2, continues to increase with distance inside the reactor in the direction of flow of the reacting gas mixture due to the fact that more reaction heat is being generated than the liquid coolant is able to dissipate. Eventually, a point is reached at which enough n-butane has reacted for the rates of reaction heat generation and of heat dissipation by the liquid coolant to equalize, such that the spatial temperature profile first flattens and then start dropping. The location at which the spatial temperature profile flattens corresponds to a point of maximum temperature inside the reactor, which is usually referred to as “hot spot”. When conditions inside the reactor are under proper control, the hot spot is located in an intermediate region of the reactor along the way of the reacting gas mixture through it. From that point on, the temperature monotonically decreases towards the gas outlet, as seen in FIG. 2.
The location and the height, i.e. the temperature value, of the hot spot can have critical effects on the reactions taking place inside the reactor. If the hot spot temperature becomes too high, structural damage of the catalyst may occur, which leads to a significant reduction of the life-cycle thereof. This typically happens if the catalyst temperature exceeds a critical temperature of about 480° C. In addition, excessive catalyst degradation due to too high temperatures may cause the hot spot to migrate towards the end of the reactor, which makes it necessary to reduce the n-butane concentration or its flow velocity, for otherwise, the gases exiting the reactor may still contain enough unreacted n-butane to cause an explosion or fire.
Conventionally, a tubular reactor for maleic anhydride production comprises a big vessel that might be as high as 7 to 10 m and have a diameter of up to 6 m traversed by several thousand tubes (typically between 5000 and 30,000) in which the reactant gases are passed and are exposed to the catalyst contained inside the tubes. The heat transfer agent or coolant is then made to flow through the vessel over the outsides of the tubes. A length of the tubes is usually referred to as “catalyst bed”.
In order to increase the reaction yield for reaction (1) to obtain more maleic anhydride, an increase in the throughput of n-butane entering the reactor may in principle be attempted. However, an increased n-butane throughput inevitably leads to more n-butane reacting according to the parallel reactions (2) to (5) as well, and hence to an increased generation of reaction heat, which results in a temperature increase inside the reactor. Further, higher temperatures tend to favour the oxidation of n-butane to CO2 and water, which is a process more exothermic than the main reaction (1). Consequently, the temperature change caused by an increase in the throughput of n-butane may lead to a higher temperature of the hot spot and to a larger fraction of n-butane reacting according to reactions (2) to (5), thereby giving rise to a smaller yield of maleic anydride.
This effect may be partly compensated by means of the catalyst activity, that is, the degree to which the catalyst accelerates the above reactions. A mixture of catalyst and an inert solid may be used to effectively dilute the catalyst in a controlled manner. Varying this mixture allows the rate of reaction in different parts of the catalyst bed to be controlled in such a way that the reduced selectivity caused by the increase in temperature can be appropriately compensated by an increased conversion rate. For example, the catalyst may be chosen to have a relatively low activity in the region where the hot spot occurs so as to minimize the magnitude thereof.
Examples of the spatial distribution of the estimated catalyst temperature along the catalyst bed, that is, as a function of height from a bottom tubesheet of the reactor, in a single reaction zone reactor are shown in FIG. 2. The direction of flow of the reacting gas mixture is represented as being from left to right. The square symbols show a case in which, as described above, the temperature starts increasing as the oxidation reaction sets on until a point of maximum temperature, the hot spot, is reached, which in the first case corresponds to a height over the bottom tubesheet of about 1400 mm and to a temperature of 450° C. The diamond symbols represent a case in which the temperature of the liquid coolant is higher than in the case corresponding to the square symbols. It can be seen that an increase in the temperature of the liquid coolant results in a higher temperature of the hot spot (around 460° C.). The change is however not significant in the higher regions of the reactor. The triangular symbols represent a case in which the catalyst is mixed with a larger amount of trimethyl phosphate than in the case represented by the square symbols. In this case, the maximum temperature, i.e. the hot spot temperature, is reduced to below 450° C. and a more gradual temperature increase in the lower part of the reactor is observed, while again, the temperature in the higher part of the reactor is hardly influenced by the modification of the catalyst.
An alternative approach was suggested in Wellauer et al., Chem.ng. Sci. Vol. 41, No. 4 (1986) pp. 765-772, according to which the process of preparing maleic anhydride is optimised by influencing the activity of the reactions through the temperature of the coolant used to dissipate the reaction heat. This was achieved by dividing the reactor into two catalyst beds or reaction zones and by independently setting two different coolant temperatures respectively. However, this alternative method did not achieve a significant increase in the reaction yield compared to the case of a reactor using a single catalyst bed and a single coolant temperature.
The method was further developed in U.S. Pat. No. 6,803,473 B2, where a process for preparing maleic anhydride in a reactor having at least two successive reaction zones cooled by independent circuits of heat transfer media or coolants is disclosed. Herein, a “reaction zone” refers to a region within the reactor in which a catalyst is kept at a controlled temperature. The temperature inside the reactor can hence be set independently in each of the reaction zones. In this part of the reactor, the temperature in the first reaction zone in the flow direction is preferably between 380° C. and 430° C., whereas the temperature in the second and further reaction zones in the flow is in a preferred range from 350° C. to 480° C., wherein the temperature difference between the hottest reaction zone and the coolest reaction zone is in any case at least 2° C. In general, the temperature is described to increase from zone to zone in the flow direction of the reacting gas mixture.
The inventors of U.S. Pat. No. 6,803,473 B2 emphasise that the yield of maleic anhydride significantly depends on the temperature difference between the hot spot maximum established in the different reaction zones, and in particular that the yield of maleic anhydride increases with increasing temperature difference between the hot spot maximum of the second or subsequent reaction zones and the hot spot maximum of a preceding reaction zone. Accordingly, they presented a reactor design in which at least one hot spot maximum of the second or subsequent reaction zones is higher than all hot spot maxima in preceding reaction zones, in particular higher than the hot spot temperature in the first reaction zone.
However, the aforementioned design has the drawback that the high temperature in the second reaction zone implies higher catalyst temperature and a larger magnitude of the hot spot therein, which negatively affects the length of the catalyst lifecycle and leads to a suboptimal reaction selectivity. In view of this, there is room for technical improvements in the design of a tubular reactor for maleic anhydride production.